Presently, the diagnosis of compartment syndrome is made by the direct measurement of intracompartmental interstitial pressure based on a technique developed by Dr. Thomas E. Whitesides, Jr. In this technique, a small amount of fluid is injected into a compartment (i.e. the volar compartment of the forearm). The pressure necessary to advance the fluid into the compartment is the measurement of the pressure of the compartment. If the intracompartmental interstitial pressure should increase to within 30 mmHg of the diastolic pressure, this could result in irreversible damage to the tissue within the compartment. The treatment for such a condition is emergency surgical release of the facia, overlying the muscle, which is constricting the compartment. Delay in the diagnosis of compartment syndrome and subsequent delay in performing the fasciotomy can result in needless loss of function, contracture, and possible amputation.
The decision to perform a fasciotomy for a suspected compartment syndrome is frequently difficult. In the classic article by Dr. Thomas E. Whitesides, Jr., "Tissue Pressure Measurements as a Determinant of the Need for Fasciotomy", Clin. Orthop., 113:43, 1975, even if physicians are well versed in the signs and symptoms of compartment syndrome, the clinical analysis sometimes is indefinite and confusing, resulting in delay in performing the fasciotomy.
According to Dr. Whitesides Jr., the one factor that must be present in a compartment syndrome is increased intracompartmental interstitial pressure. Therefore, the effectiveness of the fasciotomy is based on relieving this pressure and reestablishing tissue perfusion. In order to effectively diagnose compartment syndrome, a technique for measuring tissue pressure has been established. For details of the technique of direct intracompartmental interstitial pressure measurement, refer to the article cited above by Dr. Thomas E. Whitesides Jr. Presently, there are two electronic devices available that are based on principles outlined by Dr. Whitesides, Jr. (Ace, Los Angeles, Calif; Stryker, Kalamazoo, Mich.).
There are several disadvantages related to the Whitesides technique. First, the technique is difficult to apply continuously, second, it cannot be used to measure compartment pressure within a cast and third, it is invasive.
These disadvantages are now examined separately. First, many compartment syndromes are an evolving phenomenon. One measurement may not safely establish an ensuing compartment syndrome. A continuous noninvasive monitor of intracompartmental interstitial pressure would be very useful for assessing the possibility of an evolving compartment syndrome.
Second, for infants or those patients who have difficulty communicating, an assessment of intracompartmental interstitial pressure within a cast would be very beneficial. If pressure within the cast increases above 30 mmHg within the diastolic pressure, the cast should be removed. Obviously, the Whitesides technique which incorporates the use of a needle could not be used for detecting compartment syndrome for extremities which are within casts, unless the cast is removed. The removal of a cast, however, is time consuming and may lead to loss of fracture alignment.
Third, the invasive nature of the Whitesides technique places risk of injury to vital structures with each measurement. Puncture of the skin is painful and may also lead to infection. Finally, the invasive technique cannot be applied by non M.D. personnel such as nurses and medical assistants. With these disadvantages in mind, a noninvasive device and technique have been developed.
Compartment syndrome occurs in skeletal muscles enclosed by osseofascial boundaries. The condition develops when accumulating fluid creates high interstitial pressure within the closed osseofascial space, reducing perfusion of surrounding tissues below a level necessary for viability. As the interstitial pressure within the compartment increases, the expansion of the compartment is limited by the compliance of the osseofascial envelope. Like a balloon about to burst, the envelope becomes less and less compliant as the interstitial pressure increases. The change in compliance can be detected by palpation. The inventor, Dr. Bruce Steinberg, has quantitatively measured palpation (linear regression of force applied/to volume displaced) and has shown that a close correlation exists between this quantitative modulus of hardness and the interstitial pressure within a compartment. By measuring a quantitative hardness modulus one can noninvasively monitor interstitial intracompartmental pressures and diagnose compartment syndrome.
Another prior art device is described in Leveque etal., U.S. Pat. No. 4,159,640, which discloses a method and apparatus for measuring hardness of a material. This device comprises, "a support to rest on the material to be tested; a feeler carried by said support and arranged to bear on the said material with a predetermined force; and means for selectively and automatically detecting the displacement of the feeler only for a predetermined value P of the bearing pressure of the apparatus on the material." This measurement of hardness is inaccurate. Hardness measurements are more accurate when a modulus is formulated. The quantitative hardness modulus is the slope of the linear regression of multiple incremental measurements of force applied (y axis) to the displacements (x axis) caused by each incremental force. The device disclosed by Leveque etal, has no means of formulating a quantitative hardness modulus, only one force and displacement are measured. In order to eliminate the devastating outcome of a false negative diagnosis of compartment syndrome, or the unnecessary surgical procedure of a false positive diagnosis of compartment syndrome, the accuracy of the hardness measurement is paramount. This quantitative measurement is made accurate by formulating a hardness modulus based on multiple data points. The present invention overcomes the inaccuracy of the Leveque etal. device by obtaining a hardness modulus based on multiple data points.
Another prior art device is described in Roush etal., U.S. Pat. No. 5,038,795, which discloses a method and apparatus for the measurement of muscle tone. The device comprises a means of applying a force through a pair of calipers which pinch a muscle group (usually the superficial posterior compartment of the leg), and a means of measuring the resultant displacement of the muscle group. This device was designed for the diagnosis of medical diseases related to muscle tone, not the diagnosis of compartment syndrome. A caliper device that measures hardness is unable to be applied in the majority of compartments at risk for compartment syndrome. In the leg, for example, three of the four compartments (the anterior, lateral, and deep posterior compartments) can not be measured for hardness using the Roush device because the calipers would include the tibia bone making the measurement inaccurate. In addition, in the upper extremity, the thenar, hypothenar, carpal tunnel, and distal forearm all can not be pinched without including a bony structure. These compartments are assessed clinically for compartment syndrome by palpation not pinch. By using a device that unilaterally pushes on a muscle compartment while recording force applied to displacement measured, one can quantitatively assess hardness for all the compartments of the upper and lower extremities. The present invention overcomes the weakness of the Roush etal. device by unilaterally applying a pressure to a compartment and measuring the compression of the compartment, in effect quantitatively palpating the compartment.